The major focus of our research is to understand the genetic and molecular basis of pattern formation in the nervous system. The nervous system of multicellular organisms is a complex network of diverse cell types with unique positions and patterns of connectivity. Understanding how these intricate patterns develop is a central problem in cell and developmental biology. During the development of the nervous systems of both invertebrates and vertebrates environmental cues and cellular interactions play important roles in the determination of cellular phenotypes and in pattern formation. The compound eye of Drosophila is well suited for studying the cellular and molecular basis of pattern formation. Molecular genetic analysis of the Drosophila eye will contribute to a more complete understanding of what these developmental cues are and how they direct development. Our genetic screens for mutation affecting pattern formation in the developing eye have led us to the discovery of a new mutation. rap (retina aberrant in pattern). Our analysis demonstrates that rap gene function is critical for normal eye pattern formation. Several lines of evidence suggest that rap acts early to regulate the initial steps in ommatidial formation. First, experiments with a temperature sensitive alleleindicate that rap function is required during the third larval instar, a stage during which ommatidial pattern formation in rap mutants is abnormal immediately behind the morphogenetic furrow in the developing eye disc indicating that rap gene functions are important during initial steps. And finally, genetic mosaic studies demonstrate that rap + function is required only in R8 for normal ommatidial pattern formation consistent with the notion that rap acts at the beginning of pattern formation. These results led us to two possible models. In the first model the rap gene product is a signal made by R8 for proper patterning of other R cells. Our recent studies indicate that rap+ gene product is necessary for the proper differentiation of the R8 cell. The research proposed here is aimed at a genetic and molecular analysis of the rap gene with a view to understand the molecular nature of the rap gene product, its expression pattern and identify other gene products that with the rap gene.